In this specification the widely accepted terminology is employed with the term ‘inert gas generation’ meaning the generation of an oxygen depleted or ‘nitrogen-enriched atmosphere’ (NEA). In recent years the move towards the use of composites in the construction of aircraft wings has meant that the temperatures within the fuel tanks are greater than those in wings of conventional material due to the lower thermal conduction of the composite. Thus there is an even greater need for effective inerting of the aircraft fuel tanks in composite wings due to the greater temperatures experienced. It is well known to use one or more filters or ‘air separation modules’ (ASMs) which allow separation of a supply of inlet air into a nitrogen-enriched air portion (NEA) and an oxygen-enriched air portion (OEA). In order to run air separation modules efficiently, they need to be supplied with inlet air at a relatively high pressure (typically 40 psig (2.76×105 Pag) or more). It is possible to operate at lower pressures but this would mean that more air separation modules would be required with the consequent increase in weight and complexity, which is undesirable. By way of illustration if the air supplied to an ASM is at 15 psig, then ten ASMs would be required each weighing approximately 27 kg. But if the inlet air is at 56 psig only two ASMs are required to provide the required NEA capacity. In the past, the air separation modules have been supplied with high pressure bleed air from the main aircraft power plant. This has been bled off the compressor, cooled, filtered and then supplied to the ASM or ASMs. This system works well but there is an increasing demand on aircraft manufacturers to reduce the specific fuel consumption (SFC) of the aircraft. It is known that bleeding high pressure air from the compressor has an adverse effect on SFC and so there is now a trend to cease use of high pressure bleed air so that the engine performance can be optimised. This means that an alternative source of fluid for supply to the air separation module needs to be found and at an elevated pressure for the reasons given above.
US2006/0117956 describes an on board inert gas generation system which uses two compressors or stages arranged in series to provide compressed air to the air separation module. In order to provide high pressures to the air separation module, whilst coping with the severe strictures imposed by compressor rotor blade design limitations, US2006/0117956 provides a system in which two centrifugal compressors are run in series. The compressed air from the second stage is passed to an air separation module, but a vent is provided between the second stage compressor and the air separation module to enable the flow from the second compressor to be increased, which results in the second compressor having an increased output pressure whilst using the same compressor rotor blade design. Although this provides the centrifugal compressor with a wider operating range of output flows, it does mean that the operating efficiency is very poor at low flow rates. Since the aircraft operates at cruise during the major part of its operation, this means that for the majority of the time the centrifugal compressor arrangement is operating at well below its optimal operating efficiency.
Thus the inherent characteristics of a centrifugal compressor are ill-adapted for the operating regime and variations in the flow rates and pressures required during the cycle of ascent, cruise and descent of an aircraft and have resulted in unnecessarily complex solutions such as those set out above, which only partly tackle the issues. As noted, the ASM operates effectively at pressures above 40 psig (2.76×105 Pag). Lower pressures require a larger ASM or several ASMs (and therefore increase weight) for a given duty, whilst higher pressures may exceed the maximum working pressure of the ASM. The flow requirement for an inerting system varies with flight phase. Descent requires the maximum NEA flow-rate as the inerting system is required to re-pressurise the fuel tanks to equalize the tank and ambient pressures. Cruise requires minimum flow-rate as the NEA flow-rate is only required to make up the increase in ullage volume created by fuel burn. The ratio between maximum descent flow and cruise flow is typically up to 6:1 depending on aircraft type, cruise altitude and descent rate. This does not fit well with typical centrifugal compressor characteristics which have a very narrow flow range bounded by the surge limit and the diffuser ‘choking’ limit. In a centrifugal compressor flow can be increased by increasing speed but the pressure generated increases as the square of the speed, and the power required increases by the cube of the speed. The additional pressure must be regulated to avoid damage to the ASM. This makes it very inefficient over the flow range required by an inerting system.
By contrast, we have found that the characteristics of a positive displacement type compressor are very well suited to provide the large variations in flow, because they provide a flow rate generally proportional to speed, at a pressure sufficient to supply the pressure required by the ASM and without the substantial pressure increases at higher flow rates, which can reduce ASM life. Therefore we have designed an on board inert gas generation system which is intended to obviate some of the problems encountered with centrifugal compressor based systems.